In many wet chemical processes, a diffusion layer forms adjacent to a process surface of a workpiece. The diffusion layer is a thin region of varying material or species concentrations adjacent to the workpiece surface, and it is often a significant factor in the efficacy and efficiency in wet chemical processing. It is created by the consumption or creation of material/species at the surface. The thickness of the diffusion layer dictates the mass-transfer rate of components/reactants to the surface, and thus the mass-transfer rate can be controlled by controlling the diffusion layer. A thinner diffusion layer, for example, results in a higher mass-transfer rate. It is accordingly desirable to control the mass-transfer rate at the workpiece to achieve the desired results. For example, many manufacturers seek to increase the mass-transfer rate to increase the etch rate and/or deposit rate for reducing the length of the processing cycles. The mass-transfer rate also plays a significant role in depositing alloys onto microfeature workpieces because the different ion species in the processing solution have different plating properties. Therefore, increasing or otherwise controlling the mass-transfer rate at the surface of the workpiece is important in depositing alloys and other wet chemical processes.
One technique for increasing or otherwise controlling the mass-transfer rate at the surface of the workpiece is to increase the relative velocity between the processing solution and the surface of the workpiece, and in particular flows that impinge upon the workpiece (e.g., non-parallel flows). Many electrochemical processing chambers use fluid jets or rotate the workpiece to increase the relative velocity between the processing solution and the workpiece. Other types of vessels include paddles that have blades which translate or rotate in the processing solution adjacent to the workpiece to create a high-speed, agitated flow at the surface of the workpiece. In electrochemical processing applications, for example, the paddles typically oscillate next to the workpiece and are located between the workpiece and an anode in the plating solution.
The foregoing techniques improve the mass-transfer rate, but they may not provide sufficient mass-transfer properties for many applications. Even existing paddle-type plating tools with a series of parallel blades do not achieve sufficiently high flow velocities to adequately reduce the thickness of the diffusion layer at the surface of the workpiece in many applications. The present inventors previously developed a plating system having a series of parallel blades in which the space between the blades is completely open such that there is direct line of sight between the wafer and the anode throughout the space between the blades. The present inventors discovered that such systems may not achieve the desired flow velocities at the wafer surface for a given blade height because the agitated flows induced by the motion of such blades dissipate away from the workpiece via the open spaces. As a result, the mass transfer rate in such open-type paddle plating tools is limited.
This problem of open-type paddle plating tools significantly impairs the efficacy of such tools for plating alloys that require significant mixing to provide a desired mass-transfer rate of the ions at the workpiece. In plating alloys, the ions of one alloy element will typically have a different plating rate or bulk concentration than the other such that the alloy element having the higher plating rate may be depleted from the diffusion layer and/or more of the alloy having the higher bulk concentration will plate onto the wafer. This results in a plated layer that does not have the desired composition of alloy elements and/or is not uniform. Moreover, this problem is particularly noticeable in plating alloys or other materials into high aspect ratio features that require recirculation within the features for optimal plating results.
Existing paddle plating tools also have several other drawbacks. For example, in many existing systems the fluid flows created by the paddles do not occur in a consistent pattern across the face of the workpiece. Additionally, rotating paddles are generally not desirable in many applications because the relative velocity between a rotating paddle and the workpiece varies as a function of the radius of the paddle such that it may be difficult to accurately control radial variations in the diffusion layer at the surface of the workpiece. These problems further limit the utility of existing paddle-type plating tools in many applications.
An additional challenge of systems that hold the wafer horizontally and linearly reciprocate the paddle horizontally is that they may require large footprints to accommodate the horizontal stroke length of the paddle. In reciprocating paddle reactors, a single paddle or multiple paddle elements are reciprocated along a linear path relative to the workpiece. This may require a significant amount of lateral horizontal space within a processing tool. As a result, reactors for processing 200 mm and 300 mm wafers with horizontal reciprocating paddles are relatively large and occupy a large footprint in a tool. This is a significant drawback because floor space in fabrication lines is expensive and the operating cost of a tool is often assessed by the number of wafers that are processed per hour per unit of floor space. As a result, many conventional horizontal reciprocating paddle reactors do not efficiently use the available space within a tool.
Another challenge of wet chemical processes includes removing particulates from the surface of the workpiece or preventing bubbles from affecting plating results. Plating and etching processes can produce bubbles and particulates that become trapped under horizontal workpieces, and cleaning processes must remove particles that are already on the wafer. Many conventional systems address this challenge by inhibiting bubbles and particulates from reaching the surface of the workpiece. If particulates or bubbles become trapped under a workpiece, then flows parallel to the workpiece are required to dislodge them from the workpiece. However, it is difficult to get both a parallel flow to remove particulates and/or dislodge bubbles from the workpiece and a high velocity impinging flow to achieve high-mass transfer rates. Therefore, there is a need to provide high flow rates tangential to the surface of the workpiece.
Still another challenge of wet chemical processes is plating into openings, such as blind openings used in packaging semiconductor devices. In many applications, semiconductor dies are packaged by plating solder alloys or other metals into openings to form arrays of electrical connections on the exterior of the package. However, unless the parallel flows across the workpiece are sufficient to recirculate fluid in the openings, then the material may not plate into the depths of the openings. This can be particularly problematic in plating solder alloys because the ion species in the alloys will have different mass transfer limits such that one of the species may not plate as desired, as explained above. Therefore, there is also a need to provide higher tangential flow velocities at the surface of the workpiece than existing open-type paddle plating tools can achieve.
In light of the foregoing, it would be desirable to provide an apparatus and method for agitating the processing solution in a manner that provides controlled, high velocity fluid flows that can provide good control of the mass-transfer rates and/or high velocity parallel (e.g., tangential) flows at the surface of the workpiece. It would also be desirable to provide such agitation of the processing solution in a reactor having a relatively small footprint to increase the efficiency of the tool. There is also a need for a reactor that increases or otherwise controls the mass-transfer rate at the surface of the workpiece and provides a uniform electrical field at the surface of the workpiece.